I replaced the screen on my 6th generation iPod classic following the instructions on THIS video. However, every time I turn it on the screen is grey. Everything else works, when I plug it into iTunes everything still shows up. Any help?
One or some of the screen visual connections is loose or not connected properly and you have to go back in there. Test it before putting on the faceplate.

Those types of connections can be very fiddly. I tried a good 4 times to reconnect a laptop keyboard back on because the thing shifts so easily.
There's a good chance that what I've written above is useless and if you take any of the advice it's your own fault.